Hekla eruption 2000

The volcano Hekla is located at the intersection of the Eastern volcanic zone and the SISZ (Figure 34). Its volcanic history during historic times is characterized by one or two vigorous eruptions per century. The activity pattern changed after the 1947-1948 eruption [Þórarinsson (1967)] and smaller eruptions ocurred in 1970, 1980-1981, 1991 and 2000. Deformation measurements in the vicinity of Hekla started in 1968 with tiltmeter observations [Tryggvason (1994)]. The tiltmeter observations show a pattern of slow inflation during periods of repose and rapid deflation during eruptions [Tryggvason (1994)]. Deformation measurements have been interpreted as pressure changes in a magma source at 5 to 9 km depth below Hekla [Tryggvason (1994)], [Sigmundsson et al. (1992)], [Linde et al. (1993)], [Ágústsson et al. (2000)].

Seismic activity in Hekla is generally very low [Soosalu and Einarsson (1997)] and no long-term precursory seismicity is observed before eruptions. Short-term precursors manifested in small earthquake swarms and changes at volumetric strain stations are observed around one hour before the eruption breaks to the surface. The latest eruption started at 18:19 on February 26, and lasted until March 8. The eruption started with activity on a 4 km long fissure along the

  

Figure 32: Time series of the north components of GPS stations SOHO and HVOL assuming REYK is fixed. The start and end of the Hekla 2000 eruption are marked with vertical lines. Error bars are omitted for clarity.

  

Figure 33: Time series of the east components of GPS stations SOHO and HVOL assuming REYK is fixed. The start and end of the Hekla 2000 eruption are marked with vertical lines. Error bars are omitted for clarity.

ridge of the volcano, but confined soon to a few eruptive vents.

The closest continuous GPS station recording at the time of the eruption was at Sólheimaheiði (SOHO), at approximately 53 km distance SE of the summit of Hekla. Despite the distance a small deformation signal was seen at the stations SOHO and HVOL (Figures 12 and 17). Figures 32 and 33 show a blow-up of the time series of the horizontal components of SOHO and HVOL around the time of the eruption. The signal seems to be more prominent at SOHO and points towards Hekla. Kristján Ágústsson kindly provided the predicted displacements at the operational ISGPS stations [Ágústsson et al. (2000)]. His model of the deformation is derived from measurements at continuously operating volumetric strain stations (Figure 34). The closest volumetric strain station is 15 km from Hekla. His model is similar to the one described in [Linde et al. (1993)], consisting of a deflating pressure source at 7.7 km depth and an expanding dike with strike 65$^\circ$.

  

Figure 34: Comparison of co-eruptive signals at the ISGPS stations that were recording at the time of the Hekla 2000 eruption. ISGPS stations are marked with green circles and volumetric strain stations are marked with red circles. Legend Hekla on the map is just S of Hekla volcano. The observed displacements are shown with black arrows. Red arrows note the predicted displacements calculated from a model of the eruption based on data from the volumetric strain stations (K. Ágústsson, personal communication 2002).

In the first phase of the eruption, before the eruption breaks through the surface, the volumetric strain signal is governed by compression from a rapidly forming dike extending from the magma reservoir to the suface. In the second phase the eruption has reached the surface. The dike is still growing and pressure is decreasing in the magma reservoir. In the third phase the dike is fully grown and the volumetric strain signal shows expansion due to pressure decrease in the magma chamber. The predicted displacements for the sum of all three phases are 1.2 mm towards west and 3.2 mm north for SOHO, and 1.4 mm west and 1.9 mm north for HVOL. The displacements, estimated from the time series, at SOHO (4 $\pm$ 3 mm W and 6 $\pm$ 3 mm N) and HVOL (3 $\pm$ 3 mm W and 3 $\pm$ 3 mm N) agree with the predicted displacements regarding size and direction (Figure 34) although the signal at SOHO seems to be slightly larger than is predicted.

A new ISGPS station at Ísakot (ISAK), 15 km NW of Hekla, was installed in January 2002 over an existing benchmark that has been included in network measurements since 1986 (Section 2). During the Hekla 1991 eruption ISAK was observed to move 4.4 cm towards Hekla [Sigmundsson et al. (1992)]. Predicted displacements at ISAK for the Hekla 2000 eruption are approximately 4 cm towards the volcano.